Add blink-side binding code for navigator.serviceWorker.current

LayoutTests/http/tests/serviceworker/resources/test-helpers.js

 // Adapter for testharness.js-style tests with Service Workers
 
 function service_worker_test(url, description) {
     var t = async_test(description);
     t.step(function() {
 
         navigator.serviceWorker.register(url, {scope:'nonexistent'}).then(
             t.step_func(function(worker) {
                 var messageChannel = new MessageChannel();
                 messageChannel.port1.onmessage = t.step_func(onMessage);
                 worker.postMessage({port:messageChannel.port2}, [messageChannel.port2]);
             }),
-            t.step_func(function(reason) {
-                assert_unreached('Registration should succeed, but failed: ' + reason.name);
-            }));
+            unreached_rejection(t, 'Registration should succeed, but failed')
+        );
 
         function onMessage(e) {
             assert_equals(e.data, 'pass');
             t.done();
         }
     });
 }
 
 // FIXME: Replace this with test.unreached_func(desc) once testharness.js is updated
 // Use with unexpected event handlers or Promise rejection.
 // E.g.:
 //    onbadevent = fail(t, 'Should only see good events');
 //    Promise.then(...).catch(fail(t, 'Rejection is never fun'));
 function unreached_func(test, desc) {
     return test.step_func(function() {
         assert_unreached(desc);
     });
 }
 
 // Rejection-specific helper that provides more details
 function unreached_rejection(test, prefix) {
     return test.step_func(function(reason) {
         assert_unreached(prefix + ': ' + reason.name);
     });
 }
+
+// FIXME: Clean up the iframe when the test completes.
+function with_iframe(url, f) {
+  var frame = document.createElement('iframe');
+  frame.src = url;
+  frame.onload = function() {
+      f(frame);
+  };
+  document.body.appendChild(frame);
+}